How do i Parent a mesh to an armature that is not in rest Pose?
I got feedback from Bassam Kurdali on Blender's bf-animsys mailing lists. Here is his slightly edited answer:

There is no "bind pose" in blender. There is just a rest position and a pose. Armature deform happens based on the rest position.

the only exact solution to this problem is:


create your 'bind' pose
duplicate object to a new armature
apply the pose as new rest pose on the new armature
Use the new armature as your modifier target
Constrain the new armature to the old and use the old one as the control rig, if you desire rest pose as T pose.


One more remark: It is not possible to use the Pose library as temporary storage for the original rest pose, because the Pose library stores poses relative to the current rest pose. If the rest pose changes, the poses in the library will no longer result in the same pose as before.